Bending fatigue resistance is required for a toothed belt to be used in automobile uses such as transmission drive of an overhead camshaft (OHC), balancer drive, oil pump drive, or opening/closing drive of a slid door of automobiles. Accordingly, there is known a technology of improving the bending fatigue resistance by using a Lang-twisted cord as a tension member of the toothed belt (see Patent Documents 1 and 2). Here, Lang twist is one in which one or two or more pieces of a fiber are arranged and twisted (primary twist) and two or more pieces of the resulting one are arranged and twisted in the same direction as the direction in the primary twist (final twist).
On the other hand, the toothed belt to be used in the automobile uses are used under conditions at cold regions, for example, an extremely low temperature state of from −20° C. to −30° C., or used under highly humid or rainwater-attaching conditions. That is, the toothed belt to be used in the automobile uses is sometimes necessary to have water resistance and cold resistance while the bending fatigue resistance is secured, depending on the use environment.
For example, under the highly humid or rainwater-attaching conditions such as automobile travel at rainfall time, when the toothed belt comes into contact with water and water infiltrates into an embedded cord, the cord wetted with water swells and an adhesive component (RFL solid matter) covering around the cord separates. Then, the water promotes fatigue deterioration of the cord owing to bending, severance of the cord is caused, and thus the strength of the toothed belt remarkably decreases, so that there arises a problem that the severance of the toothed belt is likely to occur. Accordingly, there has been developed a technology of changing the cord of the toothed belt to a plied cord and improving water resistance by subjecting the plied cord to a surface treatment with a rubber cement or an overcoat (see Patent Documents 3 and 4). Here, ply twist is one in which one or two or more pieces of a fiber are arranged and twisted (primary twist) and two or more pieces of the resulting one are arranged and twisted in the reverse direction to the direction in the primary twist (final twist). The plied cord shows an advantage of little occurrence of kink as compared with the Lang-twisted cord. However, in such a toothed belt with the plied cord, there arises a problem that the bending fatigue resistance is insufficient. Therefore, a toothed belt excellent in water resistance is required while the bending fatigue resistance is secured.
Moreover, under conditions at cold regions, that is, extremely low temperature (e.g., from −20° C. to −30° C.), for example, the toothed belt is left standing in the extremely low temperature state during the automobile engine is stopped at night and a portion at which the belt is wound on a pulley is hardened at the low temperature while being bent. When the toothed belt is used in such a state, large torque is necessary for the start of the engine in the morning and the hardened toothed belt severely gets fatigue through rapid stretching and there is a concern that cracks are generated in an early period and severance of the toothed belt is caused due to extension of the cracks. Therefore, there is required a toothed belt excellent in cold resistance, which is not hardened even when it is left standing in an extremely cold state and can be started at a small torque.